Recently, fuel injection techniques have been dominant in automobile engines since the fuel injection system has a larger possibility of improving fuel consumption and exhaust emissions than other fuel systems.
Basically, there are two methods of the fuel injection in which one is an intake manifold (intake port) injection method, or a port injection method and the other is a direct injection method. In the intake manifold injection method, fuel is injected under a low pressure into an air intake conduit through a fuel injector disposed at the intake manifold. On the other hand, in the direct injection method, fuel is injected under a high pressure directly into a cylinder from a fuel injector disposed in the combustion chamber.
Particularly in the former intake manifold injection method, when fuel is injected towards the air flow in the intake manifold, air and fuel are mixed up by the turbulence of the air flow and then the mixture of air and fuel is burned in the cylinder. However, when the engine is operated at light loads, the air flow in the intake manifold is so slow in speed that the injected fuel is burned in an insufficient state of atomization in the cylinder, consequently the combustion efficiency is degraded.
In order to avoid this degraded combustion efficiency at the light loading, there have been disclosed many techniques by which the mixing of air and fuel is performed efficiently by generating a swirl flow or a tumble flow in the cylinder.
Here, the swirl flow abovementioned is an air flow rotating circumferentially along the wall surface of the cylinder and the tumble flow is an air flow circulating in the direction of the cylinder axis. It is known that the swirl flow has a large effect on homogenizing the air and fuel mixture but it has little effect on accelerating combustion by the generation of the turbulence. On the other hand, it is known that the tumble flow is effective to improve the combustion at the light loading of an engine by the strong turbulence effect which is caused when the tumble flow is broken near the end of the compression stroke.
As one example of the technology employing this tumble technology, Japanese Utility Application No. Jitsu-Kai-Hei 3-99833 discloses an intake port as illustrated in FIG. 7.
The intake port is so designed that a straight portion of the conduit is formed upstream of the intake port 1 and a center line 2 of the straight portion abuts on the valve seat 5 of the exhaust valve 4 on the sectional plane containing a center line 2 of the intake port 1.
However, in this construction, especially when an angle formed by the intake valve stem and the exhaust valve stem is small, the straight portion of the intake valve seat 3 (portion B enclosed with a circle mark) is inevitably lengthened to avoid a power loss due to a decrease of the effective section of the intake port. The result is that an air flow vector sufficient to generate a tumble flow can not be obtained.
As another example of the tumble flow technology, Japanese Patent Application No. Toku-Kai-Hei 2-301618 discloses a configuration of the combustion chamber for an internal combustion engine, as illustrated in FIG. 8 and FIG. 9. The combustion chamber is formed by four conical surfaces and respectively two intake and exhaust valves are provided on each conical surface. Further, in this prior art a ridge line dividing an intake valve side and an exhaust valve side is offset to the exhaust side so as to generate a tumble flow more easily. In this configuration a "masking effect" tends to occur at the valve seat of the intake valve 6 and further the combustion tends to become bad due to so many embosses and engraves on the surface of the combustion chamber.
As a further example of the prior art, Japanese Utility Application No. Jitsu-Gan-Sho 61-141457 discloses an induction system for an internal combustion engine as shown in FIG. 10. This prior art proposes a configuration of the combustion chamber 9 in which an intake port 8 is formed in such a way that the center line 10 of the intake port 8 is slanted more to the horizontal direction than the center line 11 of the intake valve stem and further a concave part 14 is provided at the opposite side of the intake valve 13 on the top surface of the piston According to this prior art, the tumble flow is effectively generated by the concave part 14 which is disposed on the opposite side of the intake valve 13. However, since the concave part 14 is offset against the axis of the cylinder 15, the tumble flow can not be kept alive after the bottom dead center of the piston 12.
The aforementioned prior art is concerned with modifications made to the configuration of the combustion chamber or the intake port. However, there is one example of the prior art using some additional equipment for generating a tumble flow as shown in Japanese Patent Application No. Toku-Kai-Sho 58-124019 which discloses a tumble control valve disposed upstream of the intake port in order to generate a deflected stream in the intake port. In that prior art, however, the deflected stream generated by the tumble flow control valve is weakened relatively at the early stage of generation, because there is no separate conduit only for letting flow the deflected stream, therefore, a strong tumble flow can not be expected to be generated in the cylinder.